It used to be a lot better in 7.3.5, where dungeon soloing was viable and fun, Method-0 still worked, and random queue was actually worth it. Now it's just "Quest or GTFO". And don't get me wrong here. I LIKE questing, but it was nice to have alternatives that were just as good or better. Now? Outside of a team of skilled and determined dungeon-runners, questing is hands down the best.
Honestly, if you've got the gold it's going to be a quick trip to the AH on your main, buy all the things you need/want, then mail them to your alt. IIRC the DMF Tophat can even be mailed to your own characters(can someone confirm this?), so that only requires a little bit of farming the DMF on a main. The time needed to get the hats is minor. They're cheap and easy.
I would definitely consider it worth the time to get the hats.
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It does not. I just logged on and tested it. At level 86 enemies still take a fair amount of time to drop, but don't reward good exp. At 88 they're worth even less exp, but still take enough time to kill and do enough damage to be threatening in large numbers. No more blitzing through and AOEing.